Test items and test methods for evaluating the accuracy of NC machine tools are defined in ISO 230-2, JIS-B-6201-1990, and other standards. The test items defined in such standards include errors in the amount of movement in each moving axis as well as backlash, yawing, etc. As a test method for determining errors in the amount of movement in each moving axis, for example, a method is defined that performs operations of moving incrementally by a prescribed amount along each moving axis direction, and then moving back incrementally by the same amount in the opposite direction, and then calculates the maximum value or the root-mean-square value of the errors at the respective moving points.
For the measurements of the test items such as described above, conventional contact-type gauges or magnetic scales may be used, however, laser distance measuring apparatus are more commonly used. FIG. 1 is a diagram showing an example of a prior art arrangement for measuring the accuracy of an NC machine tool (machining center) by using a laser distance measuring apparatus. As shown in FIG. 1, machine tool 91 comprises a machining tool section 92 for holding and driving a machining tool, a workpiece table 93 on which a workpiece is placed, and an NC controller 97 for controlling them. Machining tool section 92 is movable up and down (in the Z-axis direction), and workpiece table 93 is movable in two orthogonal directions in a plane perpendicular to the Z-axis direction; their movements are controlled by NC controller 97.
In the measurement of errors in the amount of movement and backlash as defined in ISO 230-2 and JIS-B-6201-1990, the actual movement amount is measured when an instruction is entered from the NC controller 97 to move in each axis direction by a prescribed amount. The illustrated example shows a case in which errors in the amount of movement and backlash in the direction indicated by arrow (X-axis direction) are measured; first, a laser light source 3 is set up so that the optical axis of the laser light emitted from laser light source 3 coincides with the X-axis direction. Next, an interference optical unit 100 constituting a part of the laser gauge interferometer is attached to the tip of machining tool section 92 so that the laser light enters the unit, and a reflective mirror (corner cube) 17 is mounted at an edge of workpiece table 93.
FIG. 2 is a diagram showing the configuration of interference optical unit 13. Laser light source 3 is a laser light source having good coherence (i.e., a long coherence length) such as a He—Ne laser, and the laser light emitted from it is split by a polarization beam splitter 131 into two laser beams. Here, polarization beam splitter 131 is set up with its optical axis oriented at 45 degrees relative to the plane of polarization of the incident laser light. In this case, the laser light transmitted through polarization beam splitter 131 is called P polarization, while the laser light reflected by polarization beam splitter 131 is called S polarization, the planes of the P and S polarizations being oriented at right angels to each other.
One laser beam (P polarization) is directed to corner cube 17 mounted on an edge of workpiece table 93, where the laser beam is reflected back in the reverse direction toward polarization beam splitter 131. The other laser beam (S polarization) is directed to a reference corner cube 132 provided in interference optical unit 100, where the laser beam is reflected back in the reverse direction toward polarization beam splitter 131. The laser beam reflected by corner cube 17 and entering polarization beam splitter 131 and the laser beam reflected by reference corner cube 132 and entering polarization beam splitter 131 overlap each other at polarization beam splitter 131, and the emergent light passes through a polarizer 138 and enters a light detector 133.
The two beams interfere with each other, forming an interference fringe whose intensity is greatest when the path-length difference between the two beams is an integral multiple of the laser beam wavelength and the smallest when the path-length difference is an integral multiple plus one-half of the wavelength. As a result, as workpiece table 93, and hence corner cube 17 mounted on an edge thereof, moves in a relative fashion, the output intensity of light detector 133 changes cyclically. More specifically, when corner cube 17 moves in a relative fashion by an amount equivalent to one-half of the wavelength, a path-length difference equal to one wavelength occurs in a round trip, the moving distance of corner cube 17, i.e., workpiece table 93, is given by one-half of the wavelength multiplied by the number of cycles in which the output intensity of light detector 133 changes.
The output signal of light detector 133 is amplified by an amplifier 134, and the amplified signal is compared in a comparator 135 with an intermediate level of the output signal and converted into a binary signal which is counted by a counter 136. A measurement value calculating unit 137 calculates the distance of movement from the value of counter 136.
Test items and test methods for evaluating the accuracy of NC machine tools are defined in ISO 230-2, JIS-B-6201-1990, and other standards, and errors in the amount of movement in each moving axis direction defined in such standards are usually measured using a laser distance measuring apparatus. In the prior art, after attaching the interference unit shown in FIG. 2 to the machine tool, the external laser light source is set up so that the laser light enters the interference unit; then, the reflective mirror (corner cube) is mounted on the workpiece table of the machine tool, and the moving distance of the table is measured. However, not only does it involve laborious procedures to set up the laser light source so that the laser light enters the interference unit in parallel to each moving axis, but there are also cases where such a setting is not possible. Japanese Utility Model Registration No. 2517929 proposes a separate-type laser interferometer that greatly enhances the freedom of setting by using an optical fiber to transmit the laser beam from the laser light source to the interference optical unit, and the above problem can be solved by using such a separate-type laser interferometer.
However, machine tools usually have three moving axes, and errors in the amount of movement must be measured for all the moving axes. As a result, when measurements along one moving axis are completed, the orientation of the interference unit and the position of the corner cube must be changed so as to make measurements along another moving axis, resulting in a problem that adjustment work is laborious and time consuming. To solve this problem, the Applicant discloses in Japanese Unexamined Patent Publication No. H09-243322 a laser interferometer that can be switched to emit a measuring laser beam in a selected one of three directions oriented orthogonal to each other.